Experiments are conducted toward understanding processes by which regulation of the immune system is controlled at the cellular level. The integrated roles of regulatory cells such as thymus-derived helper and suppressor T-cells, bone-marrow derived antibody-forming B-cells and assessory macrophages are being delineated. Specifically, we are seeking an understanding of the relationship of suppressor and helper cells and their modes of regulating antibody-forming B-cells. We have demonstrated that a streptococcal exotoxin (SPE) inhibits preferentially an activity which has been ascribed to suppressor cells. SPE is therefore potentially a useful probe for the study of suppressor cells. Subpopulations of immunocytes from T-cell sufficient plus/nu donor mice are treated appropriately with SPE or otherwise in vitro. The cells are then tested for their capacity to complement the antibody responses of T-cell deficient nude (nu/nu) mice or their spleen lymphocytes (B-cells) in vitro. This study involving SPE may also derive relevant information about the sequelae of acute streptococcal disease. BIBLIOGRAPHIC REFERENCES: Hanna, E.E. and Hale, M.L: Deregulation of Mouse Antibody-Forming Cells by a Streptococcal Exotoxin. A Model Study of the Regulation of Antibody Formation by Spleen Cells. In Battisto, J.R. and Streilein, J.W. (Eds.): Immuno-Aspects of the Spleen. Amsterdam, Elsevier/North-Holland Biomedical Press B.V., 1976, pp. 309-319. Hale, M.L. and Hanna, E.E.: Deregulation of Mouse Antibody-Forming Cells by Streptococcal Pyrogenic Exotoxin. II. Modification of Spleen T-cell Complemented Nude Mouse PFC Responses. Cellular Immunology 26: 168-177, 1976.